Plasmons Show Useful Quantum Properties

A team from the California Institute of Technology has discovered a new approach to quantum computer development, which joins plasmons, rather than photons, with waveguides. Typically, such computing relies on the creation of two photons that interfere with one another in a waveguide.

"Remarkably, plasmons are coherent enough to exhibit quantum interference in waveguides," said Harry Atwater, Howard Hughes professor of applied physics and materials science at CalTech, one of the researchers. "These plasmon waveguides can be integrated in compact chip-based devices and circuits, which may one day enable computation and measurement schemes based on quantum interference."

A plasmonic waveguide. Courtesy of CalTech.
In their study, the researchers tested for quantum interference in two plasmons by creating two waveguide paths on the surface of a tiny silicon chip. The path was contained within a 10 µm- square chip to reduce absorption.

"It should be possible to analyze these plasmonic waves using quantum mechanics in the same way that we can use it to study electromagnetic waves, like light," said Jim Fakonas, a graduate student at CalTech and a lead researcher in this study.

The waveguides, which together form a device called a directional coupler, act as a functional equivalent to a 50/50 beam splitter (used to test for quantum interference in photons), directing the paths of the two plasmons to interfere with one another. They can exit the waveguides by one of two output paths that are each observed by a detector. Both plasmons were shown to exit together, which is evidence of quantum interference.

"The quantum interference displayed by plasmons appeared to be almost identical to that of photons, so I think it would be very difficult for someone to design a different structure that would improve upon this result," Fakonas said.

Plasmons are akin to photons, and because they directly couple with light, they have been used in photovoltaic cells and other applications for solar energy. In the future, the researchers say plasmons hold promise for applications in quantum computing.

The work was funded by the US Air Force Office of Scientific Research. The research is published in Nature Photonics (doi: 10.1038/nphoton.2014.40).